Abdel Rahman, a spokesman for the Syrian Observatory for Human Rights, said: "There is a clear shift in the Isis strategy. It has moved from consolidating its total control in areas under its grip. It is now spreading.

"For Isis, fighting the regime is not about bringing down Assad. It is about expanding on control."

The reclusive leader of Isis, Abu Bakr al-Baghdadi, made a rare appearance earlier this month and issued a haunting command for Muslims across the globe to "obey" him.

Speaking in the Iraqi city of Mosul he said: "I am the wali (leader) who presides over you, though I am not the best of you, so if you see that I am right, assist me.

"If you see that I am wrong, advise me and put me on the right track, and obey me as long as I obey God in you.

"God gave your mujahedeen brothers victory after long years of jihad and patience… so they declared the caliphate and placed the caliph in charge.